This invention relates to control of an infinitely variable transmission for a vehicle.
Tokkai Hei 10-325459 published by the Japanese Patent Office in 1998 and Tokkai Hei 11-63184 published by the Japanese Patent Office in 1999, disclose an infinitely variable transmission wherein a fixed speed ratio transmission and a toroidal continuously variable transmission are combined with a planetary gear set so that the speed ratio can be varied up to infinity. The speed ratio of the infinitely variable transmission is controlled based on the vehicle speed and the speed ratio of the toroidal continuously variable transmission.
The vehicle speed is detected from the rotation speed of the output shaft of the infinitely variable transmission. The speed ratio of the continuously variable transmission is calculated from the rotation speeds of the input/output shafts of the toroidal continuously variable transmission. For this purpose, rotation speed sensors are provided respectively for the output shaft of the infinitely variable transmission, input shaft of the toroidal continuously variable transmission and output shaft of the toroidal continuously variable transmission.
The rotation speed sensor may for example comprise a gear which rotates together with a rotating body, and a Hall element which outputs a pulse according to the passage of a gear tooth, and the rotation speed is detected by counting pulses in a unit time. The output shaft of the infinitely variable transmission does not rotate when the vehicle is stationary, and therefore pulses are not counted. Also, the pulse detection interval is extremely long when the vehicle is moving at a low speed. Therefore, when the rotation of the output shaft of the infinitely variable transmission is detected by a rotation sensor having this construction, it is difficult to determine in a short time if the vehicle is stationary or it is moving at a low speed, so the speed detection precision falls in the speed region close to zero.
The infinitely variable transmission has a speciality that the speed ratio can be varied continuously including infinity. The speed ratio becomes infinity when the output shaft of the infinitely variable transmission has stopped while the input shaft of the infinitely variable transmission is rotating. In other words, the speed ratio becomes infinity when the vehicle is stationary while an engine of the vehicle is running. When the speed ratio of the toroidal continuously variable transmission is varied from this vehicle stationary state, the rotation direction of the output shaft of the continuously variable transmission, i.e., the vehicle moving direction, is determined according to the direction of the variation of the speed ratio of the continuously variable transmission. Consequently, if the vehicle speed detecting precision falls in the speed region close to zero, speed ratio control in the very low speed region including the stationary state of the vehicle may be unstable.
Further, the vehicle speed detected by the above rotation speed sensor is an absolute value, and cannot make a distinction between forward motion and reverse motion of the vehicle. When a selector lever of the vehicle is in the forward or reverse travel range while the vehicle is in the stationary state or in its vicinity, the vehicle generally needs a creep torque. When the vehicle has stopped on a rising slope and an accelerator pedal of the vehicle is released while the selector lever is in the forward travel range, the vehicle may move rearwards according to the steepness of the slope. In this case, it is desirable to rapidly detect the reverse motion of the vehicle and increase the creep torque so as to stop the reverse motion. In order to perform such a torque control, a distinction must be made between forward motion and reverse motion of the vehicle.
It is therefore an object of this invention to increase the detection precision of an output shaft rotation speed of an infinitely variable transmission in a very low speed region including zero.
It is a further object of this invention to discriminate between the rotation directions of the output shaft of the infinitely variable transmission.
It is still a further object of this invention to reduce the number of sensors required to control the infinitely variable transmission.
In order to achieve the above objects, this invention provides a controller for such an infinitely variable transmission that comprises an input shaft, a continuously variable transmission which outputs the rotation of the input shaft at an arbitrary speed ratio, a fixed speed ratio transmission which outputs the rotation of the input shaft at a fixed speed ratio, a differential mechanism having a first rotation element which rotates according to an output rotation of the continuously variable transmission, a second rotation element which rotates according to an output rotation of the fixed speed ratio transmission, and a third rotation element which varies a rotation direction and a rotation speed according to a difference between a rotation speed of the first rotation element and a rotation speed of the second rotation element, and an output shaft joined to the third rotation element.
The controller comprises a first sensor which detects the rotation speed of the input shaft, a second sensor which detects the output rotation speed of the continuously variable transmission, and a microprocessor programmed to calculate the rotation speed of the output shaft from the rotation speed of the input shaft and the output rotation speed of the continuously variable transmission, and to control the speed ratio of the continuously variable transmission based on the rotation speed of the output shaft.
This invention also provides a control method for the infinitely variable transmission. The method comprises detecting the rotation speed of the input shaft, detecting the output rotation speed of the continuously variable transmission, calculating the rotation speed of the output shaft from the rotation speed of the input shaft and the output rotation speed of the continuously variable transmission, and controlling the speed ratio of the continuously variable transmission based on the rotation speed of the output shaft.